Electron discharge device



Dec. 8; 1936. ALLEN' 2,063,360

ELECTRON DISCHARGE DEVICE Filed May 16, 1933 T 1 I I WM 0%.

INVE TOR ATTORNEY Patented Dec. 8, 1936 UNITED STATES PATENT OFFICE,

ELECTRON DISCHARGE DEVICE Application May 16, 1933, SerialNo. 671,306 2 Claims. (Cl. 250-275) This invention relates to electron discharge devices, and with particularity to an improved form of radio tube.

Heretofore, there have been employed in radio tubes two common types of electron emissive cathodes, i. e., so-called thoriated tungsten cathodes and so-called oxide coated cathodes. The thoriated cathode is characterized by the fact that it can be operated or rendered emissive at a very high temperature, while the operating temperature of the oxide coated cathode must be comparatively low. Because of this temperature limitation, it has been found very difficult to construct reliable and long-life power tubes, as high power transmitting tubes or the like, with electron emissive cathodes consisting of oxides of the alkaline earths or similar materials.

One of the serious drawbacks to the use of such oxide coated cathodes in power tubes, is the fact that such tubes, as ordinarily constructed, operate at a relatively high temperature, and their efiiciencyhas been roughly proportional to the heat dissipation of the anode. Various attempts have been made to increase the heat dissipation of the anode, for example, where metal anodes have been employed, carbonization has been resorted to.' Because of the relatively low heat dissipation of carbonizedmetal anodes, it has been found impracticable to employ oxide coated cathodes. This is particularly'true in the case of high power transmitting tubes where, in operation, the anode may be at a dull red heat. If the carbonates on the cathode have not been completely reduced to oxide form during manufacture, the high temperatures during operation may result in a decomposition of these residual carbonates, thus liberating a quantity of C02. The existence of this gas in turn results in ionization by bombardment, with consequent destruction of the cathode coating, development of bright spots, and a further increase in the amount of gas liberated within the tube.

In order to bring the cost of manufacture of such tubes to a reasonable level, the processing of the tubes is carried to a predetermined maximum temperature, beyond which temperature they are not designed to be operated without producing instability and possible inoperability. It is obvious, therefore, that with such tubes a sudden overload may result in the release of sufficient gas from the cathode coating and also from the anode and other metal parts, to render the tube useless.

It has been found that when an output or power tube is provided with a synthetic graphite anode which is preferably free from joints or seams, that it is practicable to employ an oxide coated cathode.

Accordingly, one of the principal features of this invention is to provide a power or output tube, such as a transmitting oscillating tube, with an oxide coated cathode and with a unitary anode of synthetic graphite.

It has been further found that by increasing the heat-emission efiiciency of the cathode itself, a corresponding increase in the permissible operating temperature of the tube is obtained. Accordingly, it is another important feature of the invention to employ as the carrier of the oxide coating a filament, ribbon or sleeve consisting substantially entirely of nickel and cobalt.

A still further feature relates to the novel combination of elements by means of which it is possible to construct a power tube, such as a radio transmitting oscillator or the like, having longer life and greater reliability'than ordinary thoriated tungsten cathode tubes.

Other features and advantages of the invention, not specifically enumerated will be apparent after a consideration of the following description and the appended claims. 7

While the invention will be disclosed as embodied in one type of electron discharge device, it will be understood that the invention is not limited thereto. Accordingly, Fig. 1 of the drawing is a schematic View, partly in section, of a tube embodying features of the invention;

Fig. 2 is a sectional view of Fig. 1; and

Fig. 3 shows the invention embodied in a tube having an indirectly heated cathode.

Referring more particularly to Figs. 1 and 2, the numeral I indicates any suitable form of envelope of glass or other similar material. While the drawing shows an envelope of one particular shape, it is obvious that this is merely for illustrative purposes. The envelope l is provided with a reentrant portion 2 terminating in a press 3 in which are sealed the various leading-in and support wires 4, 5, 6, l, 8. Supported on the wires 6 and 1 is a cathode 9 which may be in the form of a wire or ribbon provided with a coating of an oxide or oxides of the alkaline earth metals such as, for example, barium or strontium oxides. Preferably, the wire or filament carrying the emissive coating is of the type disclosed in ccpending application Serial No. 667,195, filed April 21, 1933, and consists substantially entirely of nickel and cobalt with the cobalt constituting from 5 to of the alloy. While the drawing shows the cathode 9 in V-shape form, it will be understood that any other arrangement or disposition of the filament may be employed, and that any well known tension means may be provided to maintain the filament taut while in operation.

Surrounding the cathode 9 is a grid I0 of any suitable foraininous structure, for example, this grid may consist of a pair of parallel support wires H, l2 around which is helically wound the grid wire, the turns of the grid being suitably fastened to the support wires. It will be understood, of course, that a mesh or wire fabric grid may be employed if desired.

Preferably, the material of the grid .is of a refractory metal such as tungsten, molybdenum, or an alloy of nickel and molybdenum.

Surrounding the grid and cathode is an anode [3 which is attached to the support wires 4 and 8. The anode I3 may be of any suitable cross section and while it is shown as cylindrical in the drawing, this is for purposes of illustration only. Preferably, however, the anode I 3 consists of a synthetic graphite prepared and processed in accordance with the disclosure of Patent No. 1,982,- 821. As disclosed in said patent, the anode I3 is prepared from graphitized amorphous carbon which has been subjected, either prior to its assembly in the envelope i or subsequent to its assembly'therein, to the action of a high or radio frequency electric field. As a result of this treatment, the gases and entrapped carbonaceous: products on the surface and within the body of the anode l3 are completely removed. As indicated schematically on the drawing, the anode I3 is preferably formed without scams or joints and if it is desired to employ a cylindrical anode, this anode may be formed by reaming, drilling or otherwise boring a cylindrical rod of synthetic graphite referred to above. It has been found that by employing a synthetic graphite anode which is free from joints, the heat dissipating properties of the tube are considerably increased. Furthermore, the fact that the anode consists of a single piece of synthetic carbon, enables the anode to be subjected to heat treatment without distorting the surface of the anode and without loosening the fastening bolts or clamps which are usually employed with plural-part electrodes.

The manner of evacuating the tube with the various electrodes above described may follow any well known procedure. For example, with the electrodes mounted within the envelope l as described, the said envelope may be connected to an evacuating pump to subject the envelope to a preliminary exhaust. The filament 9 may then be heated to a sufiiciently high temperature to re duce the carbonates to oxide form, preferably the exhaust continuing during this operation. At this stage, the anode l3 may be subjected to the action of a high frequency electric field for the purpose of driving out occluded gases and entrapped carbonaceous products, as described in said Patent No. 1,982,821. However, the anode I3 is preferably subjected to the high frequency treatment while there is air or gas within the envelope I and, if desired, as stated above, the anode 13 may be subjected to the high frequency treatment before its assembly in the tube and while it is in the open air.

It has been found that this latter procedure considerably reduces the exhaust schedule necessary in evacuating the tube.

If desired, a suitable getter or getters may be mounted within the envelope l which may be flashed after the evacuation has been completed to remove any residual gases within the tube, after which the tube may be sealed off from the pump. It will be understood, of course, that the steps described in the above processing may be varied as found most desirable. For example, the getter or keeper may be of solid or granulated form so that there is a suflicient quantity left in the tube even after the above described flashing operation.

It has been found that a tube thus constructed employing a filamentary material consisting of nickel and cobalt, and with an anode consisting of synthetic graphite which has been subjected to the action of a high frequency field in the open air, is capable of operation even with sudden overload, without subjecting the oxide coating to destructive bombardment. While this may be primarily due to the improved heat dissipation properties of the single-piece synthetic graphite anode, it may also be due in part to the increased heat vs. emission efiiciency of the filamentary material itself. It has been further found that tubes of this character are capable of being operated with greater power output than corresponding tubes employing ordinary metal or bi-part anodes and employing ordinary nickel or nickel alloy filaments.

It will be understood, of course, that the invention is not limited to a filamentary or directly heated cathode type of tube. For example, there is schematically illustrated in Fig. 3 a tube embodying an indirectly heated cathode. In this figure the parts corresponding to similar parts of Fig. 1 are indicated by the same numerals. Thus the anode 13 preferably consists of a single piece of synthetic graphite processed as above described, and the grid I0 may consist of any suitable foraminous metal. The cathode, however, in this embodiment consists of a heater filament H which is insulatingly spaced from the metal sleeve IS in any manner well known in indirectly heated cathode structures. While the drawing shows the filament l 4 as being supported and insulated by a twin bore insulator, it will be understood that this is merely illustrative. with a coating of oxides of the alkaline earths such as barium and strontium oxide. Preferably the filament I4 consists substantially entirely of a nickel-cobalt alloy of which the cobalt forms from 5 to 50% and, if desired, the metal sleeve I 5 may likewise be made from this nickel-cobalt alloy.

It will be understood that while the invention described above is in connection with three-electrode tubes, it is also applicable to two-electrode or rectifier tubes, as well as plural grid tubes such as screen grid tubes, pentodes, or the like. As is well known, natural graphite contains an appreciable percentage of amorphous carbon. The term synthetic graphite is employed herein to designate a graphite which is derived either from natural graphite or from a mixture of amorphous carbon and graphite, by subjection to a high frequency electric field whereby the finished product consists substantially entirely of crystalline carbon which has had substantially all entrapped gases and carbonaceous products removed therefrom.

Various changes and modifications may be made herein without departing from the spirit and scope of the invention.

What I claim is:

1. A high power vacuum tube, comprising an emitter of the oxide coated type, a tubular single- The sleeve I5 is provided I piece seamless anode of synthetic graphite sur- "F rounding said emitter, said anode being substantially entirely free from loose surface carbon and entrapped carbonaceous products, and an evacuated gas free envelope enclosing said emitter and said anode.

2. An anode for an evacuated electron discharge device of the type comprising an electron emitting cathode and a surrounding anode, said anode consisting of a one-piece seamless hollow tubular member of synthetic graphite, the surface of said member being substantially entirely free from loose carbon particles and entrapped carbona- 6 

